Rinse pork and pat dry. In a small bowl, mix flour, salt, and pepper; sprinkle mixture over pork. Pour oil into a 10- to 12-inch frying pan over medium-high heat; add pork and turn to brown well on all sides, about 15 minutes. Transfer pork and any juices to a slow-cooker (at least 5 qt.).
Let pan cool slightly, then add garlic and sage and stir over medium-low heat until garlic turns golden, about 1 minute. Add to the slow-cooker, along with milk and lemon peel.
Cover and cook on high until pork is tender when pierced and sauce is golden brown and reduced by about half, 7 to 8 hours; about 3 hours before pork is done, uncover the crock to let the pan juices reduce and thicken.
Transfer pork to a rimmed board and slice. Serve meat with sauce on the side.
Note: Nutritional analysis for sodium is N/A.